Hybrid vehicle architecture may take several forms for operatively connecting a battery, electric traction motor and a combustion engine together in the driveline of the vehicle. One proposed architecture in development by the assignee of this application is a Modular Hybrid Transmission (MHT). A key enabling technology of the MHT is the Electric Converter-Less Transmission (ECLT). To replicate the torque converter function of a conventional automatic transmission, the MHT powertrain relies on active controls of a starter/alternator and a disconnect clutch before the electric motor and a launch clutch after the electric motor.
Removal of the torque converter improves the powertrain efficiency, however, the drivability of the MHT must meet comparable targets to production automatic transmissions. A major control challenge of the MHT is to absorb clunks, pulsations and vibrations in the driveline during engine starts and clutch engagement, creating a quieter, stressfree driving experience.
Without the torque converter, new challenges arise as to the coordination of the clutch, engine and motor, especially during the complicated clutch engagement transients. All the friction element control, pressure control, and the motor toque control have to be integrated seamlessly for delivering smooth wheel torque. In addition, converter-less disconnect clutch engagement is very sensitive to the clutch pressure and it is a challenging task to achieve the proper damping and smoothness during the clutch engagement.
The engine in a MHT must start smoothly and quickly and every start is accompanied by a transient clutch engagement process that results in substantial inertia drags and torque disturbances that are transferred to the driveline. The difficulty and uncertainty of estimating the engine and clutch torque caused by complicated transient dynamics make the motor torque compensation a challenging task.
During the MHT clutch engagement transient for engine starts, there are problems of oscillations arising from the excitation of the mechanical resonance by various disturbances. This resultant oscillation phenomenon is due to low damping in the driveline due to the absence of a torque converter. The electric motor torque generates torque ripples with frequencies that are motor speed dependent.
The above problems and other problems are addressed by the present disclosure as summarized below.